Composite magnetic mirror and method of forming same

ABSTRACT

Disclosed is a composite magnetic mirror having a reflective surface deposited upon a glass-magnetic material composite. Also disclosed is the method for producing the composite by fusing a preformed glass blank to a magnetic material substrate.

United States P Colburn et al.

COMPOSITE MAGNETIC MIRROR AND METHOD OF FORMING SAME Inventors: StevenC. Colburn, La Palma;

Charles A. McPherson, Upland; Roger A. Miller, Montclair, all of Calif.

General Dynamics Corporation, Pomona, Calif.

Filed: July 1, 1971 Appl. 190.: 158,818

Assignee:

US. Cl. 350/1, 350/288 Int. Cl., G021: 5/20 Field of Search 350/1, 151,310,

References Cited UNlTED STATES PATENTS l2/l965 Kolk, Jr. et al. 350/l5l3/l949 Maier 350/310 Nov. 6, 1973 3,098,803 7/1963 Godycki ..204/3s1,394,085 10/1921 Halvorson ..350/310 FOREIGN PATENTS OR APPLICATIONS1,151,934 9/1958 France 350/321 OTHER PUBLICATIONS Barnes, A New OpticalBench,".l.O.S.A. Feb. 1944, Vol. 34, No. 2, pp. 110-111. Barnes OpticalBench 3 1(2 ].0.S.A. 110-111. (Feb. 1944).

Primary Examiner-David Schonberg Assistant Examiner-Michael .l. TokarAttorney-Albert J. Miller et al.

57 ABSTRACT Disclosed is a composite magnetic mirror having a reflectivesurface deposited upon a glass-magnetic material composite. Alsodisclosed is the method for producing the composite by fusing apreformed glass blank to a magnetic material substrate.

8 Claims, 2 Drawing Figures PATENIEDNUV 6 I973 3.770.335

FIG. H

INVENTORS. STEVEN c. COLBURN CHARLES A. MCPHERSON ROGER A. MILLER alum9. 7mm

ATTORNEY COMPOSITE MAGNETIC MIRROR AND METHOD OF FORMING SAME BACKGROUNDOF THE INVENTION Magnetic mirrors are utilized extensively to gflect andfocus infra-red ener Ideally, it would be desirable to famthe nir li rof a magnetic material and then simply polish the magnetic material toobtain the reflective quality required. Magnetic materials by natureare, however, to porous to provide satisfactory reflective surfaces.

In order to overcome the porosity of the magnetic materials, inserts ofstainless steel have been bonded to the magnetic material with anorganic binder or adhesive. The stainless steel can then be polished toa mirror quality to produce a surface on which the reflective materialcan be vapor deposited. While acceptable magnetic mirrors have beenproduced in this manner, the organic adhesive tends to relax with timecausing the mirror to go out of focus.

Stresses developed during the honing and subsequent machine polishingare then relieved, permitting the mirror plane to shift.

Conventional methods of applying a glass or enamel surface to themagnetic material likewise do not produce satisfactory results. While agood smooth enamel can be produced by crushing a glass frit into a finegrain powder, applying the crushed glass to the magnetic materialsuspended in a aqueous solution either in a slurry or by spraying, andthen fusing the glass to the mag netic material, the resultant enamel isextremely porous immediately under the surface. This sub-surfaceporosity, promoted by chemically combined water in the aqueous solution,hydrogen gas entrapped in the magnetic material, voids entrapped duringthe fusion of the glass, and chemical reactions at the glass magneticmaterial interface, cannot be tolerated. Subsequent polishing of theenamel exposes these pores and produces non-reflective areas.

Other standard enameling techniques, such as flame spraying of glassfrit onto the magnetic material, produce similar results. In addition tosubsurface porosity, the flame sprayed glass layer includes interstitialvoids and is subject to peeling or releasing from the magnetic material.

SUMMARY OF THE INVENTION The invention is directed to a compositemagnetic mirror in which the reflective surface is deposited upon ahomogeneous, non-porous, glass layer fused to a substrate of a magneticmaterial. A protective, scratch resistant surface may be deposited uponthe reflective surface.

The composite is formed by fusing a homogeneous, non-porous, glass blankto the magnetic substrate and then depositing the reflective surface. Bycasting a lead borosilicate glass blank and fusing it to an alnicosubstrate, a composite can be produced having a stable interface withthe glass in a state of compression at room temperature and with themagnetic properties of the alnico unaffected by the fusing. The glassretains its homogeneous, non-porous, as-cast condition which is suitablefor a reflective surface.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. 1 is a sectional viewillustration of the magnetic material substrate and the pre-formed glassblank before being formed into the composite magnetic mirror. FIG. 2 isa sectional view illustration of a formed composite magnetic mirror.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in FIG. I, thecomposite magnetic mirror basically comprises two elements, a magneticsubstrate l2 and a pre-formed glass blank 10; The magnetic substrate 12is first formed into the configuration desired for the composite mirror,such as the dish illustrated in FIGS. 1 and 2. Suitable magneticmaterials for the substrate 12 include the alnico metals, with alnico VIbeing preferred. As is generally known, alnico is an alloy containingaluminum, nickel, cobalt and iron.

The pre-formed glass blank 10 should generally have the sameconfiguration as the magnetic substrate 12. As will be explained later,considerable deviation from the substrate configuration can be toleratedand may be desired. It is important, however, that the glass have aco-efficient of thermal expansion that closely matches that of themagnetic material in the substrate. Also, the glass should have amelting and forming temperature below the temperature that wouldadversely affect the magnetic properties of the substrate.

Alnico VI has a linear thermal expansion of l 1.3 X 10- in/in/C from 20Cto 30 Q C an d its magnetic properties are not adversely affected below936 B Lead borosilicate glasses or enamels have generally loiF softeningpoints well below 930F and their c0- efficients of thermal expansion canbe significantly changed by minor additions of certain compounds.

Since the co-efficient of thermal expansion for glass is not linear butundergoes a radical change at about its equivalent rate temperature, aprecise match with the magnetic substrate is not possible. Since glassis a brittle material, and almost always fails in tension, the glassshould be designed to be in a state of compression with respect to themagnetic substrate at room temperature. This can be accomplished bydeveloping a glass that from room temperature to its equivalent ratetemperature has a slightly lower expansion co-efficient than themagnetic substrate.

The glass blank 10 must be carefully prepared to produce the desiredcharacteristics. A lead borosilicate glass having the following finalcomposition has been found to be suitable.

OXIDE RANGE NOMINAL Lead Oxide 85 Bon'c Oxide 5-l5 l0 Silicon Dioxide0-l0 5 Zinc Oxide 0-l0 5 A glass melt with the above nominal finalcomposition can be produced from the following raw materials:

RAW MATERIAL Lead Oxide Lead Monosilicate (PbO-2/3 8,0,) Boric Acid (H809 Zinc Oxide (ZnO) PARTS BY WEIGHT 9.4

to form the glass blank 10 in the desired configuration. The castingmold may be preheated if desirable. The cast glass blank can be removedfrom the casting mold and cooled slowly or left in the casting mold tocool.

' While annealing of the glass blank is not required, it

must be cooled at a rate slow enough to prevent fracturing. Ahomogeneous, non-porous glass blank can thus be produced.

The composite magnetic mirror is then formed by positioning the glassblank 10 on the magnetic substrate 12. The assembled glass blank andmagnetic substrate are then heated to fuse the glass blank to themagnetic substrate without causing the glass to flow. A stable interfacebetween the glass and the magnetic substrate can be formed by heatingthe assembly to slightly less than the glass flow temperature, at whichtemperature the glass softens or slumps but does not flow. This lowtemperature will not alter the magnetic properties of the substrate.Since the glass only slumps and does not flow, it remains quite viscousand any bubbles formed at the interface cannot migrate through theglass. The composite is then carefully annealed to relieve all stresses.While both the glass blank 10 and magnetic substrate 12 have been shownas dish shaped in FIG. 1, the glass blank could easily be a flat disc inwhich case the softening of the glass would permit it to assume itsfinal configuration. A thin film of finely ground glass frit, having asimilar composition as the glass blank may be provided between the blank10 and substrate 12 to facilitate bonding.

If desired, the glass can be ground and/or lapped to produce therequisite contour and mirror quality. Degreasing with an appropriatesolvent, followed by a thorough drying should be performed after theglass is lapped.

The reflective surface 14 can be formed by vacuum vapor depositing auniformly thick coating of a reflective material, such as aluminummetal, upon the glass surface. An overlay surface 16 such as a vacuumvapor deposited protective coating of silicon monoxide may be applied tothe reflective surface 14. A scratch resistant surface is produced byoxidizing the silicon monoxide to silicon dioxide in a warm oxidizingatmosphere. Alternately, the overlay surface 16 may be a vacuum vapordeposited coating such as magnesium fluoride. In either case, thereflective surface 14 may be cleaned, if necessary, before the overlaysurface 16 is applied.

Thus a composite magnetic mirror, capable of refleeting and focusinginfra-red energy, is produced. The glass has a thermal co-efficient ofexpansion comparable to that of the magnetic substrate and will be incompression with respect to the substrate at room temperature. Themagnetic properties of the substrate are not adversely affected duringthe forming of the composite.

While specific embodiments of the invention have been illustrated anddescribed, it is to be understood that these embodiments are provided byway of example only and that the invention is not to be construed asbeing limited thereto, but only by the proper scope of the followingclaims.

What we claim is:

l. A composite magnetic mirror comprising:

a substrate of a magnetic material;

a homogeneous, non-porous, glass layer fused to one surface of saidmagnetic material substrate; and

a relfective surface deposited upon the homogeneous, non-porous, glasslayer.

2. The composite magnetic mirror of claim 1 and in addition a scratchresistant protective surface deposited upon the reflective surface.

3. The composite magnetic mirror of claim 1 wherein said magneticmaterial is alnico and said glass is a lead borosilicate glass.

4. The composite magnetic mirror of claim 3 wherein the leadborosilicate glass is in a state of compression with respect to thealnico at room temperature.

5. The composite magnetic mirror of claim 4 wherein said leadborosilicate glass has a nominal composition of 80% lead oxide, boricoxide, 5% silicon dioxide and 5% zinc oxide.

6. The composite magnetic mirror of claim 4 wherein said leadborosilicate glass has a composition of 75-85% lead oxide, 5-l5% boricoxide, 040% silicon dioxide, and 0-10% zinc oxide.

7. A composite magnetic mirror comprising:

a substrate of an alnico permanent magnetic material;

a homogeneous, non-porous, lead borosilicate glass layer, having athermal coefficient of expansion comparable to alnico, fused to onesurface of said substrate, said glass layer in a state of compressionwith respect to the alnico at room temperature;

a reflective surface deposited upon the glass layer;

and

a scratch resistant protective surface deposited upon said reflectivesurface.

8. The composite magnetic mirror of claim 7 wherein said scratchresistant protective surface is a layer of silicon dioxide.

k i t

1. A composite magnetic mirror comprising: a substrate of a magneticmaterial; a homogeneous, non-porous, glass layer fused to one surface ofsaid magnetic material substrate; and a relfective surface depositedupon the homogeneous, non-porous, glass layer.
 2. The composite magneticmirror of claim 1 and in addition a scratch resistant protective surfacedeposited upon the reflective surface.
 3. The composite magnetic mirrorof claim 1 wherein said magnetic material is alnico and said glass is alead borosilicate glass.
 4. The composite magnetic mirror of claim 3wherein the lead borosilicate glass is in a state of compression withrespect to the alnico at room temperature.
 5. The composite magneticmirror of claim 4 wherein said lead borosilicate glass has a nominalcomposition of 80% lead oxide, 10% boric oxide, 5% silicon dioxide and5% zinc oxide.
 6. The composite magnetic mirror of claim 4 wherein saidlead borosilicate glass has a composition of 75-85% lead oxide, 5-15%boric oxide, 0-10% silicon dioxide, and 0-10% zinc oxide.
 7. A compositemagnetic mirror comprising: a substrate of an alnico permanent magneticmaterial; a homogeneous, non-porous, lead borosiliCate glass layer,having a thermal coefficient of expansion comparable to alnico, fused toone surface of said substrate, said glass layer in a state ofcompression with respect to the alnico at room temperature; a reflectivesurface deposited upon the glass layer; and a scratch resistantprotective surface deposited upon said reflective surface.
 8. Thecomposite magnetic mirror of claim 7 wherein said scratch resistantprotective surface is a layer of silicon dioxide.